The subject matter of the present invention relates to a seismic signal processing method and apparatus and, in particular, a workstation computer system, and its associated method and program storage device, which stores a novel software package known as "Variance Cube". The computer system is responsive to a plurality of seismic signals, which propagated through a cubic volume of an earth formation, for generating a cube representing said cubic volume of earth. The cube includes a plurality of seismic data samples, each seismic data sample having a corresponding "variance value" and a unique color assigned thereto. The computer system further generates one or more maps, such as a time slice map, representing one or more slices through the cube, each map displaying and being used to determine certain geologic features which exist along the corresponding slice through the cube. Each map includes a plurality of the variance values representing the geologic features, each such variance value being defined as the degree to which an amplitude of each seismic data sample in a cell in the cube at a particular reflection time "t" varies about an average amplitude of the samples in the cell.
Two dimensional seismic data is acquired along lines that consist of geophone arrays onshore or hydrophone streamers offshore. The geophones or hydrophones act as sensors which receive seismic energy from an earth formation. The seismic energy is transmitted into the earth formation, reflected back toward a surface of the earth from subsurface horizon interfaces in the earth formation, and propagates through a cubic volume of the earth formation before reaching the sensors. In three dimensional (3-D) seismic, the principle is the same except that the arrays of geophones and hydrophones are more closely spaced to provide more detailed subsurface coverage. As a result, extremely large volumes of digital seismic data are received by a computer and stored therein, the computer processing the seismic data by executing certain software stored in the computer and displaying the results of that processing. Following that processing, final interpretation of the processed seismic data can be made.
The processing of the digital seismic data requires computer resources which store and execute complex software for enhancing the received digital data/seismic signals and for muting any accompanying noise which masks the signals. Once the digital data/seismic signals are processed, the resultant processed signals are recorded and displayed in the form of a "cube" and a plurality of "maps" which represent slices through the cube, such as horizontal time slice maps or horizon maps, which display various geologic features situated on the corresponding slice through the cube. As a result, three dimensional seismic is used extensively to provide a more detailed structural and stratigraphic image of subsurface reservoirs.
During the computer processing of the seismic data, the computer responds to a set of seismic data which was digitally generated when seismic energy "sound" waves were transmitted through a cubic volume of earth. The computer operates on a cubic portion of the received seismic data (hereinafter called a "cube") which corresponds to that portion of the seismic energy that propagated through the cubic volume of earth. The seismic data in the cube comprises a plurality of seismic traces, where each trace further comprises a multitude of seismic data samples. If a horizontal plane were to pass through corresponding seismic data samples in the cube, that plane would be called a "time slice", since all the corresponding seismic data samples on that time slice have the same reflection time. Therefore, a plurality of such time slices pass through a plurality of corresponding seismic data samples in the cube (see FIG. 2). During the computer processing of the seismic data, a cell on a first time slice in the cube encompasses a first seismic data sample on the first time slice in the cube, similar such cells on other time slices in the cube encompass the same corresponding first seismic data sample on the other time slices in the cube, and a mathematical operation is performed on the seismic data samples in each of the plurality of cells on each of the plurality of time slices in the cube thereby producing a plurality of values or results corresponding, respectively, to a first plurality of the first seismic data samples in the plurality of cells of the cube. The plurality of values or results are then assigned, respectively, to the first plurality of first seismic data samples in the respective plurality of cells of the cube, one such value or result being assigned to each of the first seismic data samples. Then, the plurality of cells on each time slice in the cube move (or sequentially progress) from the first plurality of first seismic data samples to a second plurality of second seismic data samples, the above referenced mathematical operation is performed on the seismic data samples in each of the plurality of cells which now encompass the second plurality of second seismic data samples, and a second plurality of values or results is produced corresponding, respectively, to the second plurality of second seismic data samples in the plurality of cells of the cube, one such value or result being assigned to each of the second plurality of second seismic data samples. Then, the plurality of cells move or sequentially progress from the second plurality of second seismic data samples to a third plurality of third seismic data samples, and the above process is repeated until all of the seismic data samples on each of the time slices in the cube have a value or result assigned thereto. A color is assigned to each value or result corresponding to each seismic data sample. Therefore, each of the seismic data samples in the cube have a unique color assigned thereto. By slicing through the cube along the time axis, a time slice "map" is produced having a plurality of colors disposed thereon which correspond, respectively, to the plurality of values or results which further correspond to the plurality of seismic data samples on that time slice (see FIGS. 13-15). Similarly, by slicing vertically through the cube along a vertical axis, another "map" is produced having another plurality of colors disposed thereon which correspond, respectively, to the plurality of seismic data samples on that vertical slice (see FIG. 39). Consequently, the entire cube now has values or results and unique colors assigned to each of the seismic data samples in the cube and a plurality of maps can be produced which reflect the geologic features on the maps.
However, as good as the above referenced computer processing of the seismic data has become, improvements are needed. For example, there are different ways for performing the above referenced mathematical operation on the plurality seismic data samples in each of the plurality of cells on each of the slices in the cube.
For example, in U.S. Pat. No. 5,563,949 to Bahorich et al (the disclosure of which is incorporated by reference into this specification), in each of the cells on each of the time slices, a "coherency" is determined between two seismic data samples which are disposed in an "in-line" direction, and another "coherency" is determined between two seismic data samples which are disposed in a "cross-line" direction; the geometric mean of the coherency in the in-line direction and the coherency in the "cross-line" direction is determined; and that geometric mean value is assigned to one of the seismic data samples in each of the particular cells. The "coherency" is defined below with reference to FIG. 20.
In addition, in U.S. patent application Ser. No. 09/019,180 filed Feb. 5, 1998 to Peter P. Van Bemmel et al and entitled "Seismic signal processing method and apparatus for generating time slice or horizon maps in response to seismic traces and quadrature traces to determine geologic features" (hereinafter called the "Van Bemmel application"), the disclosure of which is incorporated by reference in this specification, a seismic signal trace and its quadrature trace undergo cross correlation for determining a cross correlation function (from which a plurality of values are determined for assignment to the seismic data samples) and generating the aforementioned "maps".
However, the method disclosed in the Bahorich patent (which discloses a mathematical operation for calculating the geometric mean of two coherency values to represent the value to assign to a seismic data sample in each cell on a time slice of a cube) and the method disclosed in the Van Bemmel application (which discloses a mathematical operation for calculating the cross correlation between a seismic trace and its quadrature trace) represent only two such methods and mathematical operations for generating a "cube" and a plurality of corresponding "maps" that display a set of geologic features of the earth formation in the cube.
There exist other methods for performing other mathematical operations for calculating other values or results for assignment to a seismic data sample in each of the cells on each of the slices in the cube for the ultimate generation of a "cube" and corresponding "maps" that display the geologic features of an earth formation in the cube.